HD Radio™ digital broadcasting is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. Both AM and FM HD Radio™ signals can be transmitted in a hybrid format where the digitally modulated signal coexists with the currently broadcast analog AM or FM signal, or in an all-digital format without an analog signal. In-band-on-channel (IBOC) HD Radio™ systems require no new spectral allocations because each HD Radio™ signal is simultaneously transmitted within the same spectral mask of an existing AM or FM channel allocation. IBOC HD Radio™ promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners. An HD Radio™ digital broadcasting system is described in U.S. Pat. No. 6,549,544, which is hereby incorporated by reference.
One proposed FM HD Radio™ broadcasting system uses a set of orthogonal frequency division multiplexed (OFDM) subcarriers to transmit a digital signal. OFDM modulation is a well-known technique that modulates a vector of information symbols in parallel over a number of individual orthogonally-spaced subcarriers. An OFDM signal includes multiple subcarriers modulated at different equally spaced frequencies, which are orthogonal to each other. OFDM modulation has proven to be an effective means for transmission over channels that experience various types of multipath and linear distortion. This ensures that different subcarriers do not interfere with each other under normal channel conditions.
In conventional serial modulation (not OFDM), a number (e.g. 100) of QPSK symbols (200 bits) is modulated in a sequence of 100 complex QPSK symbols over a total time interval T. In contrast OFDM groups these symbols as a vector and transmits the QPSK symbols in parallel as 100 subcarriers each modulating a single QPSK symbol. Each of the parallel OFDM subcarriers in this example occupies approximately one hundredth of the serial QPSK bandwidth and spans approximately the same time T. Both the bandwidth and throughput of the serial and OFDM transmissions are approximately the same. Small differences in the time and bandwidth are a result of channel filtering for mostly the serial transmission, and guard time (if any) in the OFDM case.
Unfiltered QPSK modulation results in a constant signal magnitude where only the phase is modulated. Therefore its Peak-To-Average Power Ratio (PAR) is one, and the power efficiency of the transmitter's high power amplifier (HPA) is high. More conventional filtered QPSK (i.e., square-root Nyquist filtering) results in a small amplitude modulation component of the modulated signal where its PAR is small (typically about 1 or 2 dB), and the HPA efficiency is high, but not as high as unfiltered QPSK.
The magnitude of the transmitted signal in such a system with OFDM modulation occasionally has very high peaks. Therefore the linear power amplifiers used in these transmitters need to operate with large power back-offs so that the out-of-band emission power is below the imposed mask limits. This results in expensive and inefficient amplifiers. For a large number of subcarriers, each complex dimension (inphase and quadrature) of the OFDM signal approaches a Gaussian distribution. This results in a signal magnitude (square root of power) Probability Density Function (PDF) that approaches a Rayleigh distribution.
Although the Rayleigh distribution has theoretically infinite peaks, the OFDM peak is limited by the number of parallel subcarriers (e.g., 100, or 20 dB), or more practically the typical peak can be limited to about 12 dB since there is little distortion in clipping the improbable tails (e.g., above 12 dB PAR) of the Rayleigh PDF. HPA power efficiency is affected since a large power backoff is required in operation to minimize peak distortion. This peak distortion not only distorts (adds noise) the subcarrier modulation, but unwanted out-of-band emission occurs due to intermodulation distortion. This leakage, being highest immediately outside the intended spectral occupancy, can be particularly difficult to suppress with filters after the HPA output. Hence, there is a need to reduce the peak-to-average power ratio (PAR) for an OFDM signal.
Several different types of PAR reduction techniques have been proposed. Some types require additional coding or phase rotation of the subcarriers. However these PAR reduction techniques require reliable side information to undo the manipulations upon demodulation, and are less attractive. Another class of PAR reduction techniques relies on an iterative algorithm to clip and predistort (or constrain) the signal to achieve the PAR reduction and suppress out-of-band emissions, requiring no additional side information. These techniques are disclosed in A. Shastri & B. Kroeger, “Method and Apparatus for Reducing Peak to Average Power Ratio in Digital Broadcasting Systems,” U.S. Pat. No. 6,128,350, Oct. 3, 2000, and B. Krongold & D. Jones, “PAR Reduction In OFDM Via Active Constellation Extension,” IEEE Trans. Broadcasting, Vol. 49, No. 3, pp. 258-268, September 2003.
This invention provides a method for reducing the PAR of electronic signals using OFDM, such as may be used in FM HD Radio™ systems.